The primate visual cortex is a layered structure, each layer containing many different types of neurons. Anatomists have distinguished about 50 distinct varieties based on differences in dendritic branching patterns, laminar position, and axonal projection. This proposal aims to extend the current knowledge of cortical anatomy to a new level by using recent cell biological techniques of tissue slice preparations, fluorescence tracing methods and intracellular dye injections. By focusing on a few specific cell populations, it will be possible to fully characterize individual cell types by their dendritic morphology, efferent projections, synaptic connectivity, and distribution within the cortex. The cells to be studied are Meynert cells and layer 4B cells. Meynert cells are the largest pyramidal cells in striate cortex, located at the layer 5/6 border. The population is probably a heterogeneous one, with some cells projecting to area MT and some projecting to the superior colliculus. Layer 4B also contains a heterogeneous group of neurons, with certain cells projecting to area MT and others to area V2. No one knows how many morphologically distinct subpopulations exist for either of these cell classes, nor how the differences in morphology correspond to the different axonal projections. To answer these questions, Meynert cells and layer 4B cells will be identified in living brain slices by retrograde transport of fluorescent latex microspheres from areas MT, V2 and the superior colliculus. The labeled cortical neurons will be injected intracellularly with Lucifer yellow, making it possible to reconstruct the dendritic and intracortical axonal branching pattern. Once the distinct cell types have been characterized morphologically, their intrinsic synaptic connections will be studied by labeling pairs of pre- and postsynaptic neurons. Labeled Meynert and 4B cells will be injected intracellularly with horseradish peroxidase (HRP), an electron dense marker. Using the branching patterns as a guide for where to inject, additional intracellular HRP injections will be made within the cells' axonal terminal fields in the same slice. Possible sites of synaptic connections will be analyzed first at the light microscopic and then at the electron microscopic level. This approach will reveal the entire dendritic branching pattern of a postsynaptic cell, and also provide quantitative information about the number and relative placement of identified synaptic connections. Finally, the distribution of these cell populations will be quantitatively analyzed from material in which cells are labeled by retrograde tracing, silver staining and/or immunoreactivity for neurofilaments, and compared with established features of cortical functional architecture. Knowledge of the morphology, synaptic connectivity, and distribution of identified cell populations within the primate striate cortex will be extremely useful in determining the functional roles played by these cells, and is crucial for understanding the functional organization of the cerebral cortex under normal and pathological conditions.